The present invention relates to a method and an apparatus for reading images by flat-bed scanning with an image sensor, and also to a slit positioning apparatus for use in an image reading apparatus.
Image lights incident on the light receiving plane of a photo-electric conversion element such as CCD that constitutes an image sensor such as a linear sensor include, in addition to those (hereinafter referred to as "direct incident light") directly incident thereon, those lights (hereinafter referred to as "flare light") which enter the light receiving plane after having been reflected from a masking surface provided within a optical window glass or near the light receiving plane of the photoelectric conversion element for protecting the element. Since the flare light enters the light receiving plane of the photoelectric conversion element at a location different from the location the image light is expected to enter, it adversely affects the obtained image.
The applicant of the present invention therefore developed a linear sensor described below in order to prevent the adverse effects of the above flare light. FIG. 1 is a perspective schematic view showing one embodiment of a linear sensor. FIG. 2 is a section thereof along the line A--A. The linear sensor comprises a casing 1, a photoelectric conversion element 4, an optical window glass 2 which contains an inert gas and which is attached to the casing 1 above the light receiving plane of the photoelectric conversion element 4 to protect and seal the same against damages and deterioration otherwise caused by dust or air. The top and the back surfaces of the optical window glass 2 are coated with protective films (e.g. of MgF.sub.2, SiO.sub.2) 11, 12 to prevent reflection of the image lights. Having thus reduced the flaring by the optical window glass 2, the image lights are converted into electrical signals by the photoelectric conversion element 4, and outputted outside by means of lead pins 3. The resultant images are of an excellent quality (refer to Japanese Utility Model Application No. 71798/1987 Laid-open No. 180948/1988)).
The above linear sensor with an optical glasss window that is coated with films for preventing reflection is capable of reducing flaring of the image lights to a certain degree, but it had its limits. For example, as shown in FIG. 3, a portion of light which is reflected by the mask 42 (such as aluminum deposited layer for intercepting light) at the light receiving section after having penetrated the optical window glass 2 and the protection layer 41 (e.g. of SiO.sub.2) of the photoelectric conversion element 4 is often reflected once again by the protective films 11 and 12 of the optical window glass 2.
Therefore, when incident lights having the grey level distribution as shown in FIG. 4 enter the light receiving plane, displacements in terms of entering points are emphasized at the borders between the shadows and highlights. As a result, the levels of output electric signals from the photoelectric conversion element will be as shown in FIG. 5. In other words, the light in the highlight is reflected and enters the light receiving plane of the photoelectric conversion element which is provided to detect the shadows, blurring the changes in density or the borders between the highlight and the shadow because of indistinct grey levels. In an apparatus which is required to read and reproduce/record images in multilevels of gradation, the reproduced images tend to be less contrasting and more ambigous if the grey levels at the borders between the shadow and highlights are converted as they are into electric signals.
The applicant of the present invention then developed another image reader to solve the problems mentioned above. FIG. 6 is a perspective schematic view of an embodiment to show the optical input system of the image reader. A light intercepting plate 110 is provided between a lighting source 101 (e.g. a fluorescent lamp, a rod halogen lamp) and a transmissive subject copy original (e.g. color reversal film) 103 at a predetermined position. The light intercepeting plate 110 has a slit 110a of a predetermined width so that the light emitted from the lighting source 101 enters the light receiving plane of a linear sensor 102 as a fine strip of light in the main scanning direction. The light intercepting plate 110 is painted or printed with a black mat coating material (such as Sunday Paint (Trade Mark) by Dainippon Toryo Kabushiki Kaisha in Japan) to absorb the light from the lighting source 101 irradiating on the portions other than the slit 110a. A strip of light coming through the slit 110 a in the main scanning direction is transmitted through the transmissive original 103 as the image light. The image light enters the light receiving plane of the linear sensor 102 via a focusing lens 104. Since unnecessary light from the lighting source is intercepted by the light intercepting plate 110, the flare light in a substantial amount can be reduced. By sequentially moving the transmissive original 103 in the auxiliary scanning direction, the entire image can be read out.
The image reader as mentioned above can produce images with good contrasts when incorporated in a system for obtaining images if the magnification is fixed. However, in a system in which at least two of the linear sensor, focusing lens and transmissive original are movable in the direction of optical axis to vary the optical magnifications, deviations in terms of the optical axis tend to occur when such components are moved. For example, when after the system is arranged as shown in FIG. 8A, the linear sensor 102 and the focusing lens 104 are moved to change the optical magnification, a displacement would occur in the auxiliary scanning direction (FIG. 8B). If this happens, the resultant image as shown by the dotted line in FIG. 8C is deviated from the position of the image that is obtained when there is no displacement in the optical axis (solid line). Similar deviations occur when displacement of the optical axis in the main scanning direction is observed (FIGS. 9A, 9B and 9C). Further, since the width of the strip of light incident in the linear sensor also varies depending on the optical magnification, the width of the slit in the above mentioned image reader must be varied correspondingly to the optical magnification. FIG. 7 shows changes in the level of electric signals at the borders between the shadow and the highlight when the slit width is varied (W.sub.3 &gt;W.sub.2 &gt;W.sub.1) at a given optical magnification. The prior art image reader was defective in that if the slit width is not adequate to the optical magnification, the resultant image lacked contrast and became quite blurred.